North American orioles are the exception when it comes to color

Kenn Kaufman’s column “ID Tips” appears in every issue of BirdWatching. In our April 2017 issue, he explains how to identify and differentiate Orchard Oriole from Baltimore and Hooded.

Among most oriole species north of the Mexican border — Orchard, Baltimore, Bullock’s, Hooded, and Scott’s — adult males are brightly patterned, while females are more cryptic. North American birders might assume this is the normal way of things, but the majority of the 30-plus oriole species live in the American tropics. In almost all of them, males and females look essentially the same. We see hints of that at our own subtropical edges. In southern Texas, the sexes look the same in Altamira Oriole and almost the same in Audubon’s Oriole. In Spot-breasted Oriole, introduced in Florida, males and females are equally bright.

So our northern species, with their different male and female plumages — exhibiting what’s called sexual dichromatism — are the exceptions to the norm. Scientists believe that the sexes of ancestral orioles looked similar, and that the duller plumages of female orioles in the north evolved later. But why?

Female orioles do more of the work of raising young, and drab plumage makes them less conspicuous to predators. But there are plenty of predators in the tropics, where female orioles still wear bright colors. A major difference is that many tropical orioles maintain a pair bond at all seasons, and both sexes defend a permanent territory. The demands of this lifestyle may outweigh the advantages of camouflage for birds that stay paired up all year. — Kenn Kaufman

Kenn Kaufman’s “ID Tips,” featuring the photographs of Brian E. Small, appears in every issue of BirdWatching. The article above is an excerpt of a column that ran in our April 2017 issue.